Patients with Hermansky-Pudlak Syndrome (HPS) have minor to moderate bleeding histories. We have examined platelet function, in particular platelet aggregation and release reaction, in these patients and found that they have deficient dense granule contents. We have measured plasma and platelet von Willebrand factor and other platelet alpha-granule constituents such as PF-4 and beta-TG in platelet lysates of HPS patients. We are also in the process of correlating bleeding times and clinical bleeding histories with platelet and plasma vWf activity and antigen levels. In the future, we plan to examine alpha-granule contents in HPS patients, employing flow cytometry.